1. Field of the Invention
The present invention relates generally to an apparatus and a method for compensating for I/Q mismatch in a Time Division Duplex (TDD) system, and more particularly, to an apparatus and a method for compensating for a gain mismatch and a phase mismatch between an I-channel (in-phase) and a Q-channel (quadrature), and a mismatch between channels that is caused by DC offset in a receiver of a TDD Orthogonal Frequency Division Multiplexing (OFDM) system.
2. Description of the Related Art
As the bandwidth of a signal used for a digital communication system is widened, an analog/digital converter (ADC) requires a high sampling frequency in order to convert a received analog signal into a digital signal. However, based on the technical limitations of the sampling frequency of the ADC and the reality of high costs in realizing the technology, an analog receiving terminal discriminates a Q-channel from an I-channel to perform quadrature-down-conversion, and performs the analog/digital conversion.
When the I-channel is discriminated from the Q-channel and quadrature-down-conversion is performed as described above, an ADC that uses a lower sampling frequency can be used in place of an ADC performing analog/digital conversion without discrimination of I/Q-channels. However, in the I/Q-channels that are quadrature-down-converted at the analog receiving terminal and analog-digital-converted in a baseband, the performance of a receiver may be reduced due to differences in a gain, a phase, and a DC offset generated during the quadrature-down-conversion.
“Quadrature receiver mismatch calibration,” IEEE Trans. Sig. Proc., Vol. 47, No. 11, pp. 3130-133, November 1999 by R. A. Green, R. Anderson-Sprecher, and P. W. Pierre, and “The Correction Of I And Q Errors In A Coherent Processor,” IEEE Trans. Aerosp. Electro. Syst., Vol. AES-17, No. 1, pp. 131-137, January 1981 by F. E. Churchill, G. W. Ogar, and B. J, Thompson have proposed methods for applying a square wave to a receiver, and estimating I/Q mismatch using a received signal converted to a complex baseband.
Also, Korean Patent Application No. 10-2000-0004053 titled “Apparatus For Estimating Channel Considering I/Q Mismatch Error And Digital Signal Receiver Having The Same” and Korean Patent Application No. 10-2002-0026853 titled “Method For Estimating And Compensating For I/Q Mismatch And Apparatus Thereof, And method For Estimating And Compensating For I/Q Mismatch And DC Offset And Apparatus Thereof” have proposed methods for estimating and compensating for I/Q mismatch using a predetermined test signal.
FIG. 1 illustrates a receiver for compensating for I/Q mismatch according to the conventional art. Description will be made using an example where a test signal is generated for estimation of I/Q mismatch.
Referring to FIG. 1, the receiver for compensating for I/Q mismatch includes a test signal generator 101, a band pass filter 103, a low noise amplifier 105, I/Q-channel mixers 107 and 108, a local oscillator 109, a phase shifter 111, low pass filters 113 and 114, ADCs 115 and 116, an I/Q mismatch estimator/compensator 117, and a reception signal detector 119.
In operation, the test signal generator 101 generates predetermined test signals in order to estimate I/Q mismatch of the receiver. The band pass filter 103 filters the test signals generated from the test signal generator 101 to pass only signals in a band used for the receiver. After that, the low noise amplifier 105 low-noise-amplifies the signals that have passed through the band pass filter 103.
The I/Q-channel mixers 107 and 108 perform quadrature-down-conversion on the signals from the low noise amplifier 105 for an I-channel and a Q-channel, respectively, to convert the quadrature-down-converted signals into baseband signals. At this point, the I/Q-channel mixers 107 and 108 receive an oscillation signal from the local oscillator 109 and a signal obtained by phase-shifting, at a phase shifter 111, the oscillation signal from the local oscillator 109 by 90°, respectively, to convert I-channel signals and Q-channel signals into baseband signals.
The low pass filters 113 and 114 filter signals from the I/Q-channel mixers 107 and 108, respectively. After that, the ADCs 115 and 116 convert analog signals into digital signals. The I/Q mismatch estimator/compensator 117 estimates I/Q mismatch of the receiver using the signals received from the ADCs 115 and 116, and compensates for I/Q mismatch of received signals using the estimated I/Q mismatch. At this point, the I/Q mismatch estimator/compensator 117 includes a separate Fast Fourier Transform (FFT) algorithm in order to estimate the I/Q mismatch.
The reception signal detector 119 receives a signal where I/Q mismatch is compensated from the I/Q mismatch estimator/compensator 117 to detect a signal.
As described above, the conventional receiver estimates I/Q mismatch using a test signal. That is, since the conventional receiver estimates I/Q mismatch using a separate test signal, the conventional receiver cannot normally receive a signal while the estimation of the I/Q mismatch is performed. Also, since the conventional receiver additionally requires the signal generator 101 for supplying a test signal to the receiver and a separate algorithm for the estimation of the I/Q mismatch is additionally realized at a demodulation terminal, additional hardware and software resources need to be assigned to realize the receiver.